The present invention relates to current source circuits and, more particularly, to a voltage controlled current source circuit.
FIG. 1 illustrates a conventional voltage controlled current source circuit 100 for generating a current in response to a voltage. Such circuits are also known in the art as voltage-to-current converters. The voltage controlled current source circuit 100 of FIG. 1 includes a transistor 101 and a load impedance 102. The transistor 101 of voltage controlled current source circuit 100 receives a control voltage Vin and produces an output current I0. The transistor 101 may be an NMOS transistor, for example. The current at the output is given by the equation:             I      o        =                  (                              V                          i              ⁢                              xe2x80x83                            ⁢              n                                -                      V            th                          )            R        ,
where I0 is the output current, Vin is the control voltage, Vth is the threshold voltage of the NMOS transistor, and R is the resistance of the load impedance.
Another conventional voltage controlled current source circuit 200 is shown in FIG. 2. The voltage controlled current source circuit 200 of FIG. 2 includes a transistor 201 and an active load impedance 202. The load impedance used in voltage controlled current source 200 is a NMOS transistor bias in the triode region of operation. The transistor 201 of voltage controlled current source circuit 200 receives a control voltage Vin and produces an output current I0. The transistor 201 may be an NMOS transistor, for example.
One major problem with voltage controlled current sources, such as those in FIGS. 1 and 2 is that the load impedance is typically implemented using integrated circuit technology. For example, the resistor in FIG. 1 may be implemented using doped polysilicon, for example. However, typical polysilicon resistors can achieve a tolerance on only about 20% across process. Additionally, process and temperature variations may cause deviations in the circuits of FIGS. 1 and 2 by as much as 60%. Accordingly, voltage controlled current sources utilizing integrated circuit resistors can suffer deviations in the voltage to current relations that are unacceptable in many applications.
Accordingly, a voltage controlled current source that reduces the variation in the voltage to current relationship across process and temperature variations is desired.
A voltage controlled current source circuit, in accordance with one embodiment of the present invention, includes a first precision reference current coupled to a voltage control node, a first voltage controlled impedance circuit having a current input and a first voltage control input, and a first current mirror having a first current terminal coupled to the current input of the first voltage controlled impedance and a second current terminal, wherein the first current mirror generates a second reference current on the second current terminal. The first precision reference current and the second reference current are coupled together at the voltage control node. Additionally, the first voltage control input is coupled to the voltage control node.
A voltage controlled current source circuit, in accordance with another embodiment of the present invention, includes a first resistor, a first current mirror having a current terminal coupled to a first terminal of the first resistor, wherein the first current mirror generates a first reference current, a first voltage controlled impedance circuit having a current input and a first voltage control input, a second current mirror having a current terminal coupled to the current input of the voltage controlled impedance, wherein the second current mirror generates a second reference current. The first reference current and the second reference current are coupled together at a voltage control node and the first voltage control input is coupled to the voltage control node.
A voltage controlled current source circuit, in accordance with another embodiment of the present invention, includes a first transistor having a control terminal coupled to a control voltage, a first terminal, and a second terminal, a first resistor, a first current mirror having a current terminal coupled to a first terminal of the first resistor, wherein the first current mirror generates a first reference current, a first voltage controlled impedance circuit having a current input and a first voltage control input, a second current mirror having a current terminal coupled to the current input of the voltage controlled impedance, and a second voltage controlled impedance circuit having a current input coupled to the second terminal of the first transistor, and a second voltage control input, wherein the second current mirror generates a second reference current. The first reference current and the second reference current are coupled together at a voltage control node and the first voltage control input is coupled to the voltage control node.
According to one embodiment, the first resistor is an external resistor for generating a reference current.
According to one embodiment, the present invention includes a method of controlling a current. The method comprises generating a first current through a resistor, generating a second current at a current input of a voltage controlled impedance, providing reproductions of the first current and the second current at a voltage control node to generate a first control voltage at the voltage control node, and coupling the first control voltage at the voltage control node to a voltage input of the voltage controlled impedance, wherein the first control voltage corresponds to the difference between the first current and the second current.
The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the present invention.